In Korean Laid-open Patent No. 2000-31978, the inventors of the present invention have disclosed the apparatus for treating wastewater comprising aerobic microbes and an upflow anaerobic reactor and the method therefor. The apparatus can treat organic wastewater containing indecomposable and toxic substances, nutritive materials (N, P) and heavy metals, and maintain high concentration of sludge stably and economically. As a reference, the conventional technique is described in FIG. 1a and FIG. 1b. 
As illustrated in FIG. 1a, the conventional apparatus comprises an anaerobic tank, an aerobic tank and a precipitate tank. Concretely, the fixed porous inflow device is installed on the lower portion of the anaerobic tank. Wastewater and returning sludge flow into the inflow device through an inject portion. FIG. 1b depicts a planar view of the inflow device schematically.
In the conventional method, the returning sludge returned by a return pump is injected to the anaerobic reactor through the inject portion and the inflow device and then stirred continuously by a mixing device so that a biased stream of sludge flowing upward and a blocking circuit phenomenon of an inflow mixture are prevented. However, there are some problems in the conventional technique. Precisely, since the inflow device is fixed and the wastewater or the returning sludge is always thrown onto a fixed region, the concentrations of the wastewater and the sludge are not maintained uniformly and the biased current is not prevented sufficiently even though the mixing device is operated.